JavaScript Web/Node APIs是否与调用栈分属不同线程？能否并行运行？

Great question—this is a core concept that trips up a lot of folks learning JavaScript's async model. Let's break this down step by step:

Absolutely. His point about Web APIs and Node APIs running on separate threads from the call stack is spot-on. Let's dive into the details to answer your full set of questions:

Do Web APIs/Node APIs run on separate threads from the call stack and event loop? #

Yes, but let's clarify the thread breakdown for both environments:

• Browser context (Web APIs): The main JavaScript call stack and the event loop live on the browser's main thread. However, built-in Web APIs like fetch, setTimeout, DOM event handlers, or requestAnimationFrame are handled by dedicated browser-managed threads (e.g., network threads, timer threads). These threads are completely separate from the main JS thread.
• Node.js context: Node's main thread runs both the call stack and the event loop. Node's asynchronous APIs (like fs.readFile, http.get, or crypto.scrypt) are offloaded to libuv's thread pool (a set of worker threads, default 4, configurable) or other specialized threads (e.g., DNS resolution threads). These operations run independently of the main thread.

Importantly, the event loop itself is part of the main thread—it's the mechanism that checks when the call stack is empty and pushes pending tasks (from API callbacks) into the stack to be executed.

Can these APIs run in parallel with JavaScript execution in the call stack? #

Yes! This is the foundation of JavaScript's non-blocking asynchronous behavior. Here's how it works in practice:

• When you call an async API (e.g., fetch('/data') or fs.readFile('file.txt')), the main thread kicks off the operation and immediately returns to executing the rest of your code (the call stack keeps running unblocked).
• The API operation runs in its separate thread in parallel—for example, the browser handles the network request in a network thread, or Node handles the file read in a libuv thread.
• Once the API operation completes, its callback function is added to the appropriate task queue. The event loop then picks up this callback and pushes it into the call stack only when the stack is empty to be executed.

One critical note: While the API operations run in parallel, the callback functions themselves are always executed on the main thread (unless you're using Web Workers in browsers or Worker Threads in Node, which create separate JS execution contexts). So your actual JavaScript code still runs single-threaded, but the heavy lifting of the APIs is done in parallel to avoid blocking the main thread.

内容的提问来源于stack exchange，提问作者Cazineer